Cycling is one sport that's known for being kind to your knees, if you do it correctly, and by correctly I mean in the manner in which your body wants you to do it. Some are natural spinners, some are natural mashers. Having said that, and without knowing your propensities, I would offer this: anytime someone new to the sport tells me they have any kind of knee pain, I tell them the same thing - ride in a lower gear at a higher cadence. Even many experienced riders won't push really big gears each season until they've built a high-mileage base of low gear/high cadence riding. Doing otherwise can risk knee injury.